The present invention relates to a method of removing moisture from a drying gas used for the drying of solid substances, in particular granular plastics. The gas, after flowing through the substance, is dried in an adsorption drying agent and thereafter heated and finally reconducted through the substance to be dried, while the adsorption drying agent, after it has reached a given moisture content, is replaced by a drying agent which has been regenerated in the meantime. The moisture laden adsorption drying agent is then, in its turn, regenerated by the application of heat.
The invention furthermore relates to an apparatus for removing moisture from a drying gas used for the drying of solid substances, particularly granular plastics, the apparatus having at least two beds of drying agent through which the drying gas flows. A drying-gas line is leading from the beds to the substance to be dried.
A method and an apparatus of the above type are described and shown in German Offenlegungsschrift DE-OS 31 45 921. In that case, two beds of drying agent are provided, one of which is flown through by the drying gas until the gas has reached a given moisture content at the outlet from the bed of drying agent; the drying gas is then conducted via a switching arrangement, through the bed of drying agent which has been regenerated in the meantime so that the first bed of drying agent which has in the meantime become saturated can then be regenerated.
As drying agents molecular sieves are generally used, for instance crystalline metal aluminosilicates or the like which are capable of absorbing moisture up to a maximum of approximately 20% by weight of the drying agent. The transfer of the moisture from the drying gas to the molecular sieves over a time period is shown diagrammatically in FIGS. 1 to 3. The transfer of the moisture takes place in the so-called mass transfer zone (MTZ). The mass transfer zone is that part of the bed of drying agent within which the moisture content of the drying agent is reduced from the inlet concentration to the outlet concentration. A freshly regenerated bed of drying agent is shown in FIG. 1. Upon increasing saturation of a part of the bed the mass transfer zone travels, as shown in FIG. 1, towards the outlet of the bed. When, as shown in FIG. 3, the mass transfer zone has reached the end of the bed of drying agent the optimum dew point is exceeded. If the drying agent, however, is utilized up to its maximum adsorption capacity (FIG. 3) there is the disadvantage that the drying gas is excessively wet at its outlet from the bed of drying agent.